The reduction of exhaust-gas emissions in motor vehicles is an important aim in the development of new motor vehicles. Therefore, combustion processes in internal combustion engines are thermodynamically optimized such that the efficiency of the internal combustion engine is considerably improved. In the automotive sector, increasing use is being made of diesel engines which, in modern embodiments, exhibit very high efficiency. One disadvantage of diesel engines in relation to optimized Otto-cycle engines may be increased soot emissions. Soot may be particularly carcinogenic owing to the concentration of polycyclic aromatic compounds; various regulations have already been introduced in response to this concern. For example, exhaust-gas emissions standards with maximum limits for soot emissions have been enacted.
Compliance with the exhaust-gas emissions standards for motor vehicles with diesel engines relies on sensors which measure the soot content in the exhaust-gas flow of the motor vehicle.
Such soot sensors measure the soot being emitted, to provide the engine management system in a motor vehicle, in a present driving situation, with information enabling it to reduce the emissions values using regulation-based adaptations. Furthermore, by means of the soot sensors, active exhaust-gas purification by means of exhaust-gas soot filters may be initiated, or for exhaust-gas recirculation to the internal combustion engine may be implemented. In the case of soot filtering, regeneratable filters are used which filter out a major part of the soot content from the exhaust gas. Soot sensors monitor the function of the soot filters and control the regeneration cycles thereof. A soot sensor may be positioned upstream and/or downstream of the soot filter, which is also referred to as diesel particle filter.
A sensor positioned upstream of the diesel particle filter increases the system reliability and ensures operation of the diesel particle filter under optimum conditions. Since this is highly dependent on the soot quantity stored in the diesel particle filter, accurate measurement of the particle concentration upstream of the diesel particle filter system, e.g., the determination of a high particle concentration upstream of the diesel particle filter, is of high importance. A soot sensor positioned downstream of the diesel particle filter enables vehicle-internal diagnostics and ensures correct operation of the exhaust-gas aftertreatment system.
The prior art presents various approaches for the detection of soot. An approach widely implemented in laboratories consists in using the light scatter through the soot particles. This approach uses cumbersome measurement apparatuses. To implement this as a mobile sensor system in the exhaust tract, use of an optical sensor in a motor vehicle is associated with very high costs. Furthermore, unresolved problems exist with regard to the fouling of the required optical window with combustion exhaust gases.
DE 195 36 705 A1 discloses a device for measuring soot particles, wherein an electric field is generated by means of the application of a constant electrical direct-current voltage between a casing electrode through which the gas flow passes and an inner electrode within said casing electrode, and the charge current for maintaining the constant direct-current voltage between casing electrode and inner electrode is measured. Good measurement results are obtained, in the context of the disclosure of DE 195 36 705 A1, if a direct-current voltage of 2000 to 3000 volts is used to generate the electric field. In the case of these electrostatic soot sensors, the current between the two electrodes changes in a manner dependent on the soot concentration in the exhaust-gas flow. The currents that arise here are however relatively small, and the current intensity thereof lies in the range from pA to low nA values. Therefore, the entire measurement arrangement for said electrostatic soot sensors must be of very high-impedance design.